PHILIPPINE INSTIT_I_ FOR D_F/-_ STUDTF-qWorking Paper 84- 07

ENVIB0_4_r/%L I_ vECTS OF _ATERSHED _V_3DIFICATI(_qS

bF

WiZfrido P. David

ENVIR0_AL _CTS OF WATERSHED MODIFICATIONS_1/

W. P. David_2/

I. Introduction

Whenever I am with a group of social and natural scientists, I

usually find myself unable to commmicate or interact effectively on the

more pressing issues on resources allocation and management. This is

partly due to the lack of con_on Vocabulary and differences in training

and experience. So I will start my paper by defining terms, hopefully,

to express my basic concepts and assumptions

A watershed or a river basin is defined by a reface line drawn

across a river or stream. All areas whose surface runoff waters pass

through this line constitute the watershed. Thus a watershed's

boundaries are defined by fixed topographic properties of the areas

upstream from the reference line. Hence, a basin may be subdivided into

smaller basins or sub-basins and vice versa.

the standpoint of resources conservation and management, a

watershed or catchment area is best viewed as a land-based ecosystem

with physically defined dimensions within which its land, _, water,

plant, a_nospheric and other components are in a state of continuous

intemaction. A watershed or its components are continuously subject

to natural and man-made modifications.

1/Paper prepared for the Sere/nat-Workshopon "Economic Policies foxForest Resources Management" sponsored by the Philippine Institute forDevelopmental Studies_held at the Club Solviento, Calamba, Laguna,February 17-18, 1984.

2/Associate Professor and Chairman, LAWREAT, CEAT, UPLB.

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Man-made modlflcatlons are for a varlety of purposes Same are

deslgned to brlng about favorable env_ro_tal mmpacts such as eroslon,

flood and water qual_ty control Others are for dlff_t reasons such

as to satlsfy basic human needs for food, fuelwood and shelter All

modlflcatlons are, however, bound to have envlronmental slde effects

The more slgnlflcant env_xonmental impacts of _tershed modlfl-

cations are hydrologxe in nature- meanlng, changes in some of the inter-

active processes In the hydrologxc or water cycle Such changes trlgger

a chain of reaetlons _n watersheds' ecosystems as water is an important

det_t of natural resources potentaals and is the major substance

of all iXfe p_oeesse_,

Figure I presents a Schematlc d1_ of the hydrologic cycle on

a typleal Ph111pplne PlVer basln The water cycle or the transfem of

water from one form _nto another goes on eontlnuouslv through the

varlous processes as indlcated by the arrc_;son Fagure 2 The total

r_noff from ralnfall (streamflow) may come from three dlff_t sources,

namely, dlrect runoff or ovemland flow, mnterflow (from soll strata

above the water table) and groundwater flow The tlme dlstrlbutlon and

quallty of runoff waters vary wlth the source or the pathway whlch the

raln water follows mn arrlv_m4_at the waterway Groundwate_ flow or

base flow is usually better In q_allty and takes longer t_e In reacbmng

the rlver network slnee it passes tbxough and gets filtered by the

unsaturated and saturated soll strata The dry season or dependable

flows from a basln ecme from groundwater storage

Overland flow as usually concentrated or b_ighintensity flow of

poorer quallty and gets LntO the waterway sooner than baseflow and

interflow Th_s runoff component serves as the prlmary process for soil

-,,3-.J•

•

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__ Atmospheric Water_M

Evapotranspiration Precipitation Evaporation

I _ , __ _ I;

_- Surface Surface runoff .or overland flow v

storage

Network of

waterways,

includingInfiltration lakes and

flood plains

---_[ Soil water Xnterflow

capillary percolationrise

[ I Iobase or ground

Groundwater water flow T tal runoff

deep percolationG

base flow

time

Figure 2. The water cycle in a river basin or catchment area.

-5-

detachment and transpor_ downslope. It is associated with floods and

ear_ies the bulk of point and non-_int pollutants into waterways and

water bodies,

The relative contributions of overland flow, interflow and base

flow to the total runoff varies with the watershed soil-cover complexes

and othem surface properties; the soil physical properties such as

intake or infiltration chamaeteristics, the ease with which the soil

stmata will allow the n_vement of water (conductivity) and the moisture

holding capacity of the uppermost soil layer; the dr_/nage network

density; human activities designed to regulate r_noff (e.g., resemvoirs,

terTaees, contour farming) and the ava/lable energy and mechanism for

dissipating water back into the atmosphere.

On the average, approximately 60 per cent of the r_infall over a

watershed is lost through evaporation and i-f_nnspimation_leaving the

remaining 40 per cent to run off its river network. The ccmbined

process of evaporation and i-_anspiration(evapotr_lnspir_tion)plays

a key role in the energy balance of a watershed. It is the pr_

mechanism for redistributing th_radiant energy incident on the water-

shed and, hence, has an interactive effect on air temperature and

relative humidity....

The human influences on the hydrologic cycle are the issues

central to watershed management. Population pressures and political

inertia force us to manipulate our watersheds without the overriding

eonc_S on the envirormental consequences of such man/pulations out-

side the basin and the degree of manipulation our basins can be

subjected to without iPr_versibly destroying their na_ capacities

for re-stabilization. The key to watershed management is relief from

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population pressures for the exploitation of more lands just enough to

implement policies whereby the pressures of eeonom/c forces and public

opinion reward the good and penalize the bad resource users. Such

policies must, however, be premised on a full appreciation on the inter-

dependence and the extent of interactions of the various components of

watersheds' ecosystems. The same is true where relief fr_n population

pressures is to be achieved through improved technology which reduces

the need for lands for producing basic human needs.

II. The Water Cycle and the Environment

Man's destiny is tied up to water. Water is the major substance

of all living organisms. It is so widespread, a good coolant and comes

close to being a universal solvent. It is capable of storage, diversion

and transport. Moving water can possess enough energy to drive water

turbines, detach and transport soil particles as well as dissolve and

transport non-point pollutants. Thus the movement of water through the

hydrologic cycle influences the physical, chemical and biological

parameters or factors of our environment.

The effects of the water cycle on the physical properties of our

envircnment stem largely on the key moles that water plays in a land-

scape's energy balance, in weathering rocks and determining the eventual

nature of the soil that iS formed, in soil erosion and sediment trans-

portation and in the formation of landscapes. Differences in these

physical properties or processes are due largely to the non-uniformities

in the spatial and temporal distributions of water in its various forms.

Too much concentration of rainfall and runoff, for example, results in

floods which favor higher soil erosion rates, more gullies and water-

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ways, and water loving vegetation. Too little rain water at ce_tain

periods, on the other hand, may adversely affect canopy type and density,

air temperature and the rate of topsoil formation.

The human influences on the space and time distribution of water

and on these physical properties are many and varied. The most important

ones are: land use transformation or vegetative cover modifications,

land trea_ts such as terraces, contour ridges, mulches, furrrywsand

tillage, installation of drainage and irrigation systems, and surface

water storage modifications.

Moving water carries solids, bases and other liquids in solution.

Essential life process elements such as nitrogen, carbon and sulfur are

volatile and soluble and, hence, can move throughout the water cycle.

Runoff waters may carry essential plant nutrients, mineral salts and

toxic elements in solution. Rain water may also pick up certain gases

and solids in solution. Toxic elements coming from farm and industrial

wastes and high mineral content may affect the suitability of water

for agriculture, industries, domestic and wildlife conservation.

In forest and agricultural watersheds_ the mome significant modi-

fications influencing chemical properties are land use transformations

favoring intensified farming systems which increase farm chemicals

usage, feedlot runoff, agricultural wastes, installation of irrigation

and dr_age systems that may lead to salt built-up, acid sly]fatesoil

conditions_ mining operations which increase the hazards from toxic

chemicals like boron getting into waterways and construction of water

impounding structures that interfere with the usual supply of plant

nutrients and the leaching out or dilution of the concentration of the

mineral contents of downstream waters.

-8-

Regardless of its form, water supports a multitude of living

organisms. At any given time, this biological cc_munlty is in a

delicate state of ecological equilibrium. Even with small variations,

the quality and quantity of flowing water can upset this delicate

balance or equilibrium.

On the surface of the soil thrive not only the vegetative cover and

wildlife but also a wide variety of microorganisms. These organisms

break down organic materials to produce humus which is ultimately

reduced into a mineral form. The action of these microorganisms not

only influences soil fertility status but also affects the various pro-

cesses 4m the hydrologic cycle. Humus not only has a high water

retention capability but has a high infiltration rate. Higher infil-

tration rate implies less overland flow, more groundwater recharge and,

hence, lower flood peaks and soil erosion and higher base or dependable

flows.

Fresh water, whether in storage or in transport, is the habitat

for complex plant and animal forms. From the headwaters of a watershed

to shallow farm ponds, main waterways, lakes, marshes and estuaries, a

particular population of complex organisms abounds. A given population

exists in a state of precarious balance that depends primarily on

cc_petition for food and the quality of water and forms a part of the

food chain of the aquatic ecosystem. Any disturbance on the water cycle

may result in the destruction or rapid growth of another type of

organism that may be pathogenic. Such•a disturbance could occur from

one or a combination of the following: increased in point and non-point

pollutants moving within the hydrologic cycle; change in land use; change

in the time and space distribution of rainfall and runoff; con_ereial

II

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exploltatlon cf some forms of aquatlc organlsms, surface storage modl-

flcatmons such as r_servolrs, flood contr¢l and _rlgatlen infrastructures

and flshponds and land treatment such as ter_aclng, t±llage and sell

chemical amelloratlon

The above consmderatlons on the relatlonsh_ps between the water

cycle and the human envlronment focus only on the more mmportant effects

resultmng from n_ndlflcatlonsof watersheds and themr water cycles

It is not possible tc llst down much less dmscuss all the foreseeable

envlronmental mmpacts Moreover, tmme and space eenstralnts will not

allow me to deal wlth the above mentloned reldtlonshlps between the

water cycle and the human envmronment Hence, the following dlscusslons

w111 be focused further on the mmpacts of the more _portant forest

watershed modlfleatlons on the water cycle and the physleal envlrenment

III Influence of Land Use on Watershed Behavlour

A Forest Lands

1 Influence of forests on creclp_tatlon

There ms a wmdespread bellef that forests increase preclpltatlon

There are, however, no theoretlcal basls nor expe_ntal _vldences

to thls hypothesms It is now generally agreed that forest, in

comparmson to other covers, does not affect slgnlfleantly the raln-

fall over a catchment

The prmnar_yeffects of forests are on the lnterceptlon and

dlslrlbut_on of ralnfall mnto the sell system In areas wlth llght

and frequent ralnfall, a forest cover dlsslpates a eonsldem_ble

amount of ralnfall through the evaporation of intercepted rain

water and delays the arrlval of rsan water mnto the sell surface

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Evaporation from a wet forest c:mnopyproceeds at the potential or

maximum rate thus decreasing the amount or volume of rainfall for

soil moisture storage and runoff. The retarding effect of a forest

cover on rainfall provides more time for infiltration and increases

the time for runoff thus decreasing flood flows and increasing

interflows and base flows.

2. Influence of forests on infiltration

With the exception of few isolated man-made modifications

such as deep tillage with contour ridges formation and mulching,

the infiltration rate of the soil under a forest cover is greater

than any other type of land use. High infiltration rates implies

a reduction in surface runoff and soil erosion as well as increased

groundwater recharge. In areas with distinct wet and dry seasons,

however, groundwater withdrawal of deep rooted forest plants for

evapotransp/ration n_]ynegate the favorable effects on the base

flow of the increase in groundwater recharge.

3. Influence of forests on runoff

The most favorable and dramatic impacts of fbrest cover are

on the time distribution of runoff or runoff hydrograph, quality of

runoff water and the protection of soil on erosion. A runoff

hydr_graph pmoperties of interest ame usually the peak or flood

flows, the base or low flow_% and the total runoff volume or water

yield.

Ther_ are numerous experimental evidences documenting the

effects of forest cover on streamflows. Table 1 sunmmrizes the

proportions of r_/nfalls with 10-year return periods that directly

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Table 1. Direct runoff coefficients of small watershed'1/

Percentage of 10-year rainfall that goesWatershed cover or into direct runoff.or overland flowL_Iland use

f

1.0 3.0 6.0 10.0

1. Hardwood forest 0.02 0.12 0.16 0.22

2. Permanent pasture(well managed) 0.02 0.16 0.25 0.33

3. Wheat crop-withimproved soil and"water management 0,20 0.23 0.24 0.25practices

4. Second year meadow 0.33 0.38 0.43 0.45

5. Corn- with improvedpractices 0.52 0.59 0_64 0.68

6. Corn- prevailingpractices 0.68 0.71 0.72 0.73

!/Adopted from Schwab, et al, 1966. Soil and Water ConservationEngineering. John •Wiley and Sons, Inc. New York.

2/For small watersheds with moderately high runoff potentials and shallow,clay-textured soils.

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runs over the soil surface in small watersheds of varying vegetative

cover. An increase in flood flows-of several order of magnitude

may result by converting forest into meadows or crop lands.

The most extensive studies (_ov, 1962 and Bockhov, 1959)

on the effects of forest cover on streamflows has been conducted

in the U.S.S.R. where long term observations on more than 2000

highly-instrumented plots and more than 100 watersheds of vary/rig

sizes are fully documented. .Theirfindings clearly demonstrated

that forest cover yields mere than agricultural lands as the

combined effects of storage and infiltration exceeded those of

increased evapotranspiration by forest. The flood flows when

expressed in terms of the flood magnitudes to the average annual

flows or coefficient of flood flow showed the following inverse

relationship between floods and forest cover.

Coefficient of flood flow = A- 0_009F

where A is the flo_d flow eoeffic/ent for open areas and F is the

proportion of the basin covered by forest.

There are also indications in the literature to the effect

that the removal of some of the protection offered by fcmest tend

to increase flood flows. Major forest fires in small and medium

size watersheds in Australia showed a six-fold increase in the

projected flood flows (based cn the long-term streamf.lowrecords

of the basin prior to the burning of the forest cover) resulting

from high intensity storms, (UNESCO, 1972).

Clear cutting of forests have also been shown to reduce base

or low flows, increase the water table in areas with permeable soils

and increase stream temperatures (MoGuiness and Harold, 1971; Brown

and Krygier, 1970 and Bettenay, Blac_nore and Hingstone, 1974).

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In the Philippines, there is a dearth of information on the

effects of forest cover on streamflow. Preliminary studies by _he

author on several river basins in the island of Mindoro gave some

qualitative indications on the impacts of land use on the low flows

of small and medi'_msized catchments. Table 2 sh3ws that the levels

of dependable flow per unit catchment area are lower in watersheds

with less forest cover and high percentage of badly disturbed

(Savannah) lands. The same trends were observed by the author in his

Table 2. Specific flows and land uses in three Mindoro watersheds.

.......Bucayao Bugsuanga Cagumay

km2Catchment area, 384 438 146

Specific dependable flows inm3/sec/km2, catchment

80% dependability level 0_052 0.006q 0.005090% dependability level 0.037 0.0041 0.0036

Major land useCategories in %

Forest 45 16 16Cultivated area 33 18 10

Grasslands (good stand) 4 45 19Savannah (cogon and talahib grasseswith shrubs and brusher) 18 18 54

r i ....

analyses of the dependable flows of many catchments in Southeast Asia

in connection with his participation on the feasibility analyses of

irrigation projects.

The long term wet and dry season flows of a small tributary

of the Pampanga river give us some cause for alarm as far as human

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influences on small watersheds are concerned. The basin is small

enough (150 square kilometer) that the effects of land use transform-

ations are readily manifested. The conversion of forests and lowland

rice paddies into urban and cultivated areas resulted in increased

flood flows and decreased low flows as shown on Figures 3 and 4,

respectively. At some point in time, the river ceases to be

perennial and became ephemeral. Analysis of rainfall at the nearest

rain gaging station showed no significant changes in the rainfall

patterns. There were also no significant changes in the total water

yield to indicate u_nmnitored diversions in and out of the basin.

An analysis of the streamflows of a few large river basins having

long series of records did not reveal any significant changes in

their flow patterns. It may be that the impacts of modifications on

larger watersheds are manifested more slowly.

4. Influence of fforests on soil erosion and sediment transport

Soil erosion has been identified by the National Environmental

Protection Council (NEPC) as our most serious envirorm__ntalproblem

today. More than half of the Philippine land area has been either

badly eroded or is prone to severe soil erosion as a result of

unplarne(_massive land use transformations, high rainfall intensity,

unfavorable topography and shallow top soil. One does not have to

look far to see widening circles of badly denuded lands and, with

justifieation_ most of us associate these with the destruction of

our forests.

Soil erosion is a chain of the interrelated process of soil

detachment by rainfall and runoff and downslope transport by these

•same processes. Soil detachment and transport by rainfall are

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8 -- _ Basin: Pampanga River Basin7- ii Gaging Station: Maasin River,6 I Dil/man _ San

5 i-. !I Rafael, Bulacan

I

4 J Ii t\

! /. i

3__

' t t s

2 . l _ h I

'i 'l i \\ Ii. _ i I 1 "

¢oo,ooo_ _ i i_ ' I

8_ / J '_ _ 17_ I l6 l ' _ , i

-- I j' _l " v / / t! ' v I I

July' i l 1 l3--/ __-_ _ I

v \

i0,000-- August

9 --8 -7 _

6 -

5--

4 --

,

_

1,000-- __ .

_-6_-1-7._8_49J 5U[b,l_.[b'.2.J53154155156157158159 160 161162 [63 !64 !65 !66 !_?!6!J_j _0-]'7'1 ]

Year 19J ,

•Figure 3o Maximum discharge for July and August forMaasin River

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BASIN: PA_N_ RIVER BASIN

Gaging Station: Maasin River, Diliman, San Kafael, Bulacan

1000-_-9-8?-6-

5-

4-

3- / \1

/ \

' / \

k '4-, / v,-I

.._. !

•,H _ k.

IApril

YearL

Figure 4. Minimum monthly discharge for Mmmch andApril from Maasin river.

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largely influenced by the erosivity or the energy of falling rain-

drops, the erodibility of the soil, slope and the soil cover. The

detachment and transport capabilities of runoff, on the other hand,

depend primary on the depth of overland flow, the flow rate and

velocity, the hydraulic gradient of both overland and channelized

flow, soil factors and the presence of soil stabilizing agents such

as plants' rOot systems.

Soil erosion is of many forms. The uniform r_moval of a thin

layer of soil over the watershed surface is known as sheet erosion.

It is due mostly To raindrops splash and overland flow detachment

and t_ansport. The movement of soil particles may only be for a

short distance before they are deposited downslope or all the way

into the waterways and the mouth of the watershed.

The irregularities on the surface of a watershed result in the

non-uniform concentration of overland flows. The overland flow

may be concentrated on small depressions or rills. The resulting over-

land flow scour is termed as rill erosion. As rill erosion progresses

unabated, deeper incisions or _gulliesmay be formed. Gullies are

usually formed on headwaters of established wat_ys.

The scouring and sediment transporting capabilities of flowing

water in channels or water ways depend on the flow depth_ velocity

and magnitude. Empirical evidences showed that the suspended sediment

carrying capacity of a river is directly proportional to its discharge

raised to a power anywhere from 1.4 to 2.0. A two-fold increase in

river discharge will, for example_ increase the sed/ment carrying

capability of a river from 2.6 to 4.0 times. This explains the fact

that for most rivers in the Philippines (characterized by large

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variations in their discharges), more than 90 per cent of their

sed/ments transported are aLL_ibuted to the extreme 5 per cent of

the flows.

The.major effects of forests on soil _xDsion are in the cushioning

of raindrop impact, reductions in the magnitude of overland and river

flood flows as a result of the high water retention and infiltration

capacities of forest soils and the stabilizing effects of tree-r_ots

on wat_xways. Although there are other means of achieving these

effects (mechanical methods of erosion control), forests have the

added advantages of producing t/tuberand forests by-products and pre-

serving /mportant fauna and flora.•

Although numerous studies have•been conducted on the rates of

soil erosion under various covers in the Philippines, the results from

such studies are of very limited .use or value to watershed resources

planners and policy makers.. For example, the .results of the two most

cited studies are shown on Table 3. The reported erosion rates in

the first study are unrealistically low while those in the second

study are quite high (especially those measured on undisturbed areas,

coffee and banana areas which usually provide good canopies or cover).

The large discrepancies in the reported soil loss rates and the

obvious lack of accuracy in most of the local studies may be traced

to the following reasons.

1) Instrumentation errors. Very often, the measuring instru-

ments used disturb the surrounding soil, are not properly calibrated

or served an added function of controlling soil erosion. This is

more so in case of plot experiments where physical boundaries are

erected thus modifying the slope and slope length. The weirs, flumes

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Table 3. Effects of forest covers on soil loss rates.

r , ,

Cover Erosion rate, tons/ha/yrr i

A. Kellman (9.969)

Primary forest 0.09

Softwood fallow 0.13

Imperata or eogon grassland 0.18

New rice kaingin 0.38

12-year old kaingin 27.60

B. Veracion and Lozez (1979) on kaingin crops

Pineapple 308.0!I/

Coffee 318.0

Tiger grass 396.0

Castor bean 36.0.0

Banana 414.0

Banana/coffee/pineapple intercrops 421.0

Undisturbed areas 251.0

1/Converted from _n/yr byassuming a soil bulk density of 1.0.

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or flow dividems that are used to measure flows disturb the natural

flow conditions to such an extent that runoff water is impounded thus

modifying the slope and slope length and promoting the deposition of

suspended sediments.

More than others, the above reason explains the apparent

inaccuracies in the monitored erosion rates in the two long-term

studies on soil erosion in the country. In one study, severel

erosion plots with various covers were established in various locations

all over the country. For a given location and a given cover, the

soil loss rates did not correlate well with rainfall and runoff

/ (R2<0.4). In the other study, it was reported that "slope and land

use have no effects on soil erosion rates". This finding tends to

or simply violate the fundamental laws of physics and hydraulics.

2) lack of control on many important parameters affecting soil

erosion. Usually, only one or two parameters (e.g., rainfall and

cover) are monitored without due consideration or control on the

other parameters (e.g., slope, slope length, evapotranspiration,

infiltration and overland flow).

3) Lack of understanding on the part of the researchers of the

mechanics of soil erosion, the hydrologic forces at work and open

channel hydraulics. Depending on the point of measurement within

the watershed, the measured soil loss may only be sheet erosion or

only part of sheet erosion (after deposition within the waterway)

plus gully and river bank erosion.

An extensive review by the authom on the reported erosion rates

in both temperate and tropical countries showed that it is feasible

to make some rough or first round approximations on the erosion

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rates under various cover, slope and rainfall conditions in Philip-

pine watersheds. Using a modified universal soil loss equation of

the form

A= RxKx SIx CxP

wher_

A : Sheet and rill _ 'eroslon rate in tons/ha/yr

R = index of z_infall energy which is derived frcm analyses

of runs of daily rainfall intensity

K = index of soil erodibility which depends mainly on soil

texture and degree of aggregation

SI = slope index = aSn + b

a,b and n = constant

S = slope

C = index of soil cover and

P = conservation practice factor which was assumed to

be equal to unity

estimates were made on the sheet and rill erosion rates over the

407,700 hectare Magat reservoir watershed.

In the above equation, the cover index C was assigned a range

from zero to 100 to account for the full variations on the effects

of cover on soil erosion. The estimated values of C are as shown

on Table 4. Five r_nfall stations with long series of daily rain-

fall records were used to delineate 5 subareas or rainfall polygons

within the watershed. Six slope ranges and three soil erodibility

categories were also mapped over the entire watershed.

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Table 4. Relative influences of cove? on sheet and rill erosion

expmessed as indices of cover (C) in the modified soilloss equation for the Magat reservoir watemshed.

I I _ I I " ] _ ' I ]

Covem .. C valuer L. '- f |_

Prinmry forest (with _m_ll patches of clearings) 0.10

Secondary fore,st(with i patches of shmubs,clear/ngs and plantation crmps) 0.70

Grassland, good cover, unburned, ungrazed annualupland crops with terrace 0.80

Established coconut_ cacao_ coffee and fruit _orchards I.0

Rangeland with good management, improved pastures,_s with 3 to 5 year old trees, establishedipil-ipil stand i.5

Annual crops like corn, pinkie, field legumesin good rotation or inter_ropping, i-3 yearold ipil-ipil stands or banana plantation 4.0

Monoeulture of corn, field crops, pineapple newkaingins, slightly ov_zed range land 8.0

Overgrazed _s, old kaingins, _ partlydisturbed by earth moving operations(e.g._ road construction) 25.0

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The thlessen polygon_ slope, land use and soll erodaballty

index maps _ supemlmposed to generate a map show_ng annual sheet

and r111 er_sicn rates, The varlous subareas on The eroslon maps

were plan_tered and the weaghted erosion rates fc_ the varxous

categomles were calculated Table 5 presents the estamated mean

eros/on z_tes for the va_lous land use eateSorles The dlsilzebed

(pru_vy femests excluded) have a w_ghted mean of about

39 tons/ha/yea# For the ent&re w_tershed_ the mean sheet and r111

eroslon rate _Is estlmated at 29 tons/ha/y#

To check on the aecun_cy of the eroszcm rates estamates and for

/ofthe purpose cahbrmttng or _ef_unE the nu_&cal values assumed

to the varlcus eroslon _ndlces &n the equation c_ model, 36 suspended

sed_ant load _a_ts In the Magat flyer were analyzed and a

suspended sedament rating curve was demlved From the daaly stzmam-

fl_s, the annual sedlment loads for a 10-year perlod were calculated

The bed loads of the Magat tavernweme asstmed to be 25 per cent of

thesuspendedload

The results showed a mean annual Total suspended load of

23 5 tcas/ha/ym wbach was less than the estamated sheet eroslon

rate of 29 0 tons/ha/yr Consldem_ng the fact that gully erosaon has

yet to be added to the sheet and r111 eroslon estamates, it seems that

the exoslon model over_stamates by at least 25 per cent the actual

SOll eroslon loss

A closer investlgatlon of the rlver network showed that there

iS conslde_le se_t deposltlon wlT_hLnthe watel-waysalonE the

lonE stretches of the Magat flyer and its tr&Buta_les upstream from

the suspended sedlment gaging polnt In scme areas, the flyer bed

- 24 -

Table 5. Estimated mean sheet and rill erosion rates by majorland use categories in the Magat watershed.

Soil loss rate

Land Use tpns/halyr

I, Pr_ forest (with small patches ofclearings) 8.0

2. Secondary forest (with patches of shrubsand small clearings) 12.0

3. Open @rasslands 84.0

Hillside farms or kaingins (100)

Overgrazed a_eas (250)

Pastures unglazed and slightly

grazed (48.0)

Newly reforested areas (30)

Alienable and disposable a_easof mixed land uses (48)

4. Cultivated areas near settlementsand along flood plains 6.4

I) Lowland rice paddies (1.8)

2) Diversified upland crops (48.0)

Weighted average 29.0

- 25 -

was r_ised by as much as 4 meters from its original level. Fresh

mounds of river washings and islands are also being continuously

formed indicating that the rate of soil erosion far exceeds the

carrying capacity of the Magat river in the meantime. A physical

manifestation of this fact is the constant meandering of the Magat

rive_. At one location, its waterway has meandered by as much as

5 kilometers.be

The main conclusion that can/drawn from this study is that as

far as the Magat watershed is concerned (where there are available

data from experimental areas on the effect of cover on soil erosion),

the erosion model seems to yield fairly good r_sults. The good

thing about the model is that it essesses the impacts of various

lan@ uses in relation to soil, climatic sad other topographic pro-

perties. It offers watershed planners and land use policy makers

a wide range of land use options in managing watershed resources

subject to the constraints on soil erosion_ climate_ topography and

other fixed watershed properties.

The relative rankings and magnitudes of the cover indices on

soil erosion shown on Table 5 are probably accurate to within 70 to

80 per cent as far as the present conditions in the Magat watershed

are concerned. Forest stands and conditions of open areas vary from

one watershed to another. Much remains to be done by way of refining

these estimates and consldeming variations in canopy stands or

conditions if the method is to be adopted tc other watersheds.

5. Influence of forests on air temperature

The presence or absence of forests, especially where the altem-

native land use is bar_ soil or buil-up area may affect air tempe-

- 26 -

rature because of the removal of the cooling effect of evapotrans-

piration. Close to 600 gin-caloriesof heat is dissipated in every

cubic centimeter of water that is evaporated. Also, the amount of

incoming radiation that is reflected back (albedo) into space vary

with surface cover. The average albedo of a forest covem (20-25%)

is slightly greater than that of annual crops (15-20%), meadows

(15% and moist bare soil (10-20%).

Bo Grasslands

From the viewpoint of soil and water conservation, the term

"grasslands" is a very loose definition as it attex_ptsto consider

a complex group of soil cover with a broad range of management

praetiees into a single uniform land use. Every hydrologist, however,

knows only toe well the wide range of variations in infiltration,

runoff, evapotranspiration and erosion beb2.viorwhich arise from

manage_nentpractices. In some cases, grasslands are the natural

products of deforestation. Without exploitation fom other uses such

as food crop and livestock produetion_ the cover is transitional in

nature representing an initial state /n forest regeneration. In same

exceptional cases, grass is considered as a major crop and is culti-

vated, fertilized, irrigated_ mowed and managed for opt/mum forage

yield thereby rendering the term grasslands hydrologically deceptive.

Managed grasslands can not be differentiated from crop lands.

In _ost Philippine watersheds, grasslands are seldom left

unexploited or managed. As a large proportion of these areas are

governmental lands, there is a natural tendency to exploit them

without regards to the environnmntal consequences. There are notable

- 27 -

exceptions to this trend, however. Certain ethnic groups in the

Philippines have t_aditional concer_ for land and the envi_orment

and tend te practice integrated and time-lzeiedconservation and manage-

ment practices for open areas. Some users who have long-term lease-

held contracts manage their holdings for economic and posterity

reasons.

The hydrologic effects of grasslands practices depend largely

on the climate and level of managenmmt. The effects of a well managed

grassland; providing all year round or permanent cover closely

resemble those of a good forest. This type of grassland USlm]ly has

high infiltration capacity and provides a very high degree of soil

protection. The other typ_s of grasslands that are not over_azed,

regularly burned and mi_ged have effects on the hydrologic cycle

and the env/monment which are in between those of a forest and these

of cultivated annual crops. The relative influences of these land

use types have been discussed in the section dealing_with the influence

of forests on the environment.

.Mismanagedor overexploited grasslands tend to produce ephemeral

streams, high flood peaks and large amounts of soil erosion. Eventually

some grass species (e.g., imperata_ themeda) which can take advantage

of unfavorable water regimes and soil fertility conditions become

dominant. Left by themselves, these dominant grass species t_nd to

gradually improve the soil infiltration chlractemistics and afford

good soil protection.

An interesting study on the hy4rologic responses of the two most

dominant grass species in the Philippines has recently been conducted

using field plots, clay pots and 24 large, drainage type lys/meters

- 28 -

(Bueno, 1980). These two grass species proved e_-remely resistant to

sul_nergeneein water and cL_ought, Continuous flooding for more than

40 days did not significantly reduce their bicmass _ction. They

also survived a sustained moisture stresses of mor_ than 15 bars.

Hence, they can provide permanent cover even in very harsh soil and

climatic conditions as long as they are not regularly burned and over-

grazed.

Both grass species can thrive on heavily compacted soils with

very little ae_ation_ In fact, they dominate other gmass species by

initially compacting the soil, This is more so in the ease of cog0n

which develops a dense under,round root system with rhizc_es, The

natural release of nutrients frr_nthe soil and the action of organisms

On the decaying organic materials eventually improve the soil chemical

and physical properties which would enable other plant species to

survive.

Undisturbed stands of•both grasses give excellent soil protection

against sheet erosion. The runoff peaks in cogonal areas are, however,

slightly higher than those of other grass species. Hence, cogon stands

tend to favor gully erosion.

The livestock carrying capacity of grasslands depends on climate,

level of management and soil productive capability. A major reason

for overgrazing is the failure to consider the natural variability of

•climatic parameters such as rainfall. There is a tendency to increase

the population of livestock whenever a sequence of good climate occurs,

with the consequence of extensive overgrazing in the inevitable dry

years. Severe 0vergrazing results in heavy soil compaction, decreasing

infiltration, falling water table, high erosion and sediment transport

and, consequently, pressures to expand areas for pastures.

- 29 -

Again, there are indirect evidences showing the interrelated

effects of climate and grazing conditions. Table 6 presents the

monitored mean annual suspended sediments tramsported by several

rivers in the island of Mindoro. The eastern side of the island

(Mindoro Oriental) is characterized by high and more uniformly distri-

buted rainfall while the western side have a very z_ronounceddry

Table 6. Est£mated annual suspended sediment loads for selectedriwms in Mindoro.

_n |rm-i

River Catchment area, Mean annual sedimentyjn2 transport _ tonS/ha

A. Mindorr_Oriental

1. Pula 161 4.6

2. Bongabon 369 6.73. Bucayao 300 4.0

B, Mindoro Occidental

1. Mamburao 144 438.2

2. Pagbahan 337 171.03. Busuanga 415 233.3

r _r n

season lasting for 4 months or more. The watersheds on the western

side of the island have over 60 per cent of their areas in pastures

or open grasslands. This is approximately twice as much that for the

watersheds in Mindoro Oriental. The more favorable climate of Mindoro

Oriental makes crop cultivation more viable. Higher production levels

also lessen the population pressures to exploit forests. Among ckhers,

these factors help explain the better conservation and land utilization

practices in the province.

- 30 -

The watershed of western }lindo_oare severely ove_grazed. The

results are severely eroded lands and annual sediment loads that are

sevemal orders of magnitude higher than those in Mindoro Oriental.

C. A_able Lands

As mentioned earlier, the conversion of forest lands into vege-

tations with shorter growth durations and status tends to decrease

evapotranspiration and lessens the infiltration rates of soils. Thus

clearing of trees for field crops production (e.g., cereals, field

legumes, root crops and industrial crops) will result in higher over-

land flows, flood flows _nd erosion rates. A notable exception to

these trends is conversion to lowland rice _ddies. Lowland rice is

grown m_ldersubmerged conditions in level, bench terraces that trap

rain water and conserve the soil_

The removal of a _hiek forest canopy and the cultivation of soil

for seeding inevitably results in the loss of natural flow regulation

and protection of raindrop ene__gy. There are mechanical means,

however, of simulating these beneficial effects of forests. These

inmlude brcad based and bench terraces, farm ponds, gully plugs,

contour farthingand mulching. The r_st effective of these are

terraces. They are, however, usually associated with high investment

costs and unfavorable economic returns. Even with more than 50 per

cent government subsidy, the payback period to individual far_er

investors in United States, for example, is more than 30 years.

,- 31 -

D_ Urbanization a_d Industrialization

Urbanization affects the water cycle by the changes that it

triggers on the soil infiltration and surface Dropertles and on water-

sheds' energy balance and mieroclimate. Urbanization an@ indus-

trialization may also _ve marked effects on the quality of water in

the hydrologic cycle.

Urban infrastructures are usually impervious to water and hence,

reduce the infiltrating surfaces. The effects are.increased runoff

and daily va_iatlons in temperalure. Large thermal g_adients and the

increase in aff_nosphericparticles that serves as condensation nuclei

for rainfall tend to increase precipitation° The available information

at present indicate that urbanization increases the number of thunder-

storn_ which may increase the total precipitation over an area by as

much as 10 per cent.

Urban and industrial areas are gener_lly populated by people

with divergent life styles and increasing standard of living. These,

in turn, result in increasing demaIldfor water and increasing volumes

of domestic and industrial wastes w?_ch pollute land surfaces, moving

and sta_nant water and the atmosphere.

IV. The Influences of Other Man-Made Modifications on Watershed Behavior

A. Evaporation and Evapotranspiration Suppression

The effect of the use of anti-transpirant, shading, etc, to

reduce transpiration and the use of .mulches,plastic covering

and soil additives on water yield and low flows is the same as that

of reducing the amount of vegetation. Although mulches Clearly

reduce erosion, there are no significant evidences as to whether

- 32 -

those practices affect peak flows and ground water quality and

quantity (Larson, 1971).

B. Land Treatments

Terraces, contour ridges, furrows, tillage operation and mulches

are mostly designed for soil erosion and water control. Field

studies have shown that these practices re.sultin the reduction of

water yield and flood peaks _nd the subsequent increase in ground

water storage, low flow, and surface water quality. In watersheds

with level terraces, the streamflows tend to be more uniformly dis-

tributed over a long period of time (Harold and Dragoun, 1969,

Shockey, et al, 1970). _nese effects are more pronounced in very

small watersheds and diminish with watershed size (Richardson, 1972).

There is little if any evidence as to whether these practices

influence groundwater quality.

C. Installation of Drainage and Irrigation Systems

Surface drainage systems shorten the time base of flood hydro-

graphs. Subsurface drains may result in a longer time lag of a

storm hy(Irograph. They also strongly influence low flows and, to

some extent, the water quality. Substmface drains reduce low flo_

volumes. Flow thrmugh subsurface drains is usually poor in q_lity.

The effect of an irrigation system within a basin is usually

localized in nature excent for the return flow. large scale irri-

gation from ground water supply may, however, lead to a sharp

reduction in base or low flows.

- 33-

D. Surface Storage CO_sldematmons

The modification of surface storage will have pronounced

effects on water yield, flow rates and surface water quality.

Two of the main functions of a reservoir are fleod control and low

flow augmentation. A reservoir usually traps about 85 percent of

the combined bed-load and suspended sediment load of a waterway.

Reservoirs, lakes, and farm ponds also affect the water table depth

and stream temperatures (Williams, 1971).

Other modifications like importing or exporting water,

intensive recreational use and urbanization can have far reaching

hydrologic effects especially on the time and spatial distributions,

water quality and quantity. With these modifications, however, a

distinction between water use and consumption must be made. Water

used, say, for domestic and industrial purposes may be re-used

for other purposes like irrigation. Water consumed, say, by

plants, may be considered as water loss.

There are other possible man-made modifications such as ground-

water recharge, construction of infiltration galleries, desilting

basins, cloud seeding, etc. An understanding of these and their

hydrologic effects is basic to the protection of watersheds land

and water resources_

- 39 -

V. Sunmmry and Conclusions

The Philippines covers some 30 million hectares. The Philippines

has never been any smaller_ It will never be any bigger.

In 1920_ there were only 10 million Filipinos. Today, there are

about 50 million of us. The Ph/lippines is still of the same size.

Its land and water resources are bound to feel the strains from our

needs and wants.

It has been estimated that less than 35 pernemt of the country's

land area is still covered with forests. The rest has been disturbed

and transformed into other la_d uses.

Of great envir_nmental conce/_nare the badly disturbed grasslands,

uplands and pasture areas, These represent approximately one-third

of the country's land area at present and increase in coverage at the

rate that is about eq_-nlto the population gro%_h rate as rural dwellers

move into ever steeper slopes in search of food, fuelwood and shelter.

It is qu/te obvious that the root cause of all our emviro_tal

problems today is the rapid iO_owthin population. Many of our long

term policies and programs for natural resources development, control

and management are becoming obsolete or socially unacceptable before

they are fully realized because of the doubling of the human loads of

our watersheds every 2 or 3 decades.

It is equally obvious that to be effective_ a watershed management

scheme must be pr_mised on satisfying the basic needs of the people

in and around the watershed and in allocating,the natural, financial

and technical resources on the basis of their individual and economic

_ell being.

- 40 -

At thls point, it should further be stressed that no two watersheds

are allke Slm!lar]y, no t_ sets of human populatlon will response

in exactly the same manner to a glven watershed management scheme

Each watershed must be assessed o_nrefullyand given cemDetent technleal

plannmng of multlple land uses that are vltal to the present and future

welfare of its h_mmn conmunltles

Thls paper _seussed *he relevant envmronmental mmpacts of water-

shed mDdlfleatlons with the obDeetlve _f provldlng sound physleal bases

for land use pollemes for rural and forest watersheds It is hoped

that so!l ercsle_n,water qual]ty and other hydrc_neteorolog±caleoneerns

be _{ivenFroper conslderatlon in thls present and future nmnagement

pollcy maklng exerclses

- 41 -

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